An attempt will be made to elucidate the biological significance of the protein modification system (PMS) in relation to its possible role in cellular aging using human fibroblasts. This PMS is unique in that amino acids are transferred from aminoacyl tRNA to the NH2-terminal end of polypeptide by the specific aminoacyl tRNA transferase resulting in post-translational modification of proteins. It was found that the population doubling time and cellular protein content increase as the cells enter phase III, whereas the activity of arginyl tRNA transferase and the ability of chromosomal proteins to accept arginine at the NH2-terminal end were diminished progressively during cellular senescence. Furthermore, the above mentioned parameters, which are associated with cellular aging in non-transformed cells, remained fixed at the time when the cells were exposed to SV40 viruses. In order to examine fuuther the fixation of cellular senescence by SV40 viral transformation an attempt will be made to characterize fibroblasts transformed by tsA mutants of SV40 viruses at the permissive and non-permissive temperatures. We will first determine if those tsA transformed human fibroblasts have a finite lifespan at the non-permissive temperature. We will then try to study the cellular senescence associated parameters using fibroblasts transformed by tsA SV40 viruses at the permissive and nonpermissive temperatures.